Traumatic injury of the ankle joint involving injury to the collateral ligaments is one of the most frequently occurring injuries at the lower limb. These injuries most frequently occur during a variety of athletic activities. With the number of participants in sports activities rapidly on the rise, it is expected that the clinical diagnosis and treatment of such injuries is becoming a growing problem. The major goal of the proposed work is to assist in solving this problem by developing a quantitative diagnostic technique for identifying the damaged ligaments in the chronically unstable joint. The proposed technique is based on the hypothesis that different combinations of damaged collateral ligaments have different unique and identifiable effects on the load-displacement and flexibility characteristics of the ankle joint complex. Based on the validity of this hypothesis, the damaged ligaments can be identified by comparing the load-displacement and flexibility characteristics of the chronicaly unstable joint to those of the intact contralateral joint. To achieve the goal of the project, the following procedures will be adopted. An experimental system for determining the three dimensional load-displacement characteristics of the ankle joint will be developed. This system will be based on measuring the displacement of the calcaneus with respect to the shank in response to controlled forces and moments applied to the foot. The system will be utilized to determine the load-displacement and flexibility characteristics of the chronically unstable ankle and of the intact contralateral joint in thirty patients with unilateral chronic ankle joint instability. The difference between the characteristics of the chronically unstable joint and those of the intact joint will then be compared to the effect of artificially induced ligament injuries on the characteristics of cadaveric human ankle speciemens (as obtained previously in Part I). Finally, the difference between the mechanical characteristics of the chronically unstable joint and those of the intact contralateral joint will be correlated with the site and extent of the ligament injury as determined by: 1) patient's historical data; 2) physical examination; 3) radiographic and MRI findings; and 4) surgical findings.